#1781: Type equality class leads to non-termination
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Reporter: chak | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.9
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Os: Unknown | Testcase:
Architecture: Unknown |
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Comment (by chak):
This program was rejected by 6.6.1 with the message
{{{
/Users/chak/Code/haskell/E_Class.hs:10:0:
The equation(s) for `plus' have two arguments,
but its type `Int -> a' has only one
}}}
which is arguably not the right thing to do either.
I haven't traced this, but the problem may well be due to an interaction
between equality constraints and FDs - at least since I fixed
`TcUnify.subFunTys`. The latter function will defer a wanted equality `a
~ (b1 -> b2)`, which in principle would be entailed by the given `E a (Int
-> Int)` (with `b1, b2 := Int`). However, the normalisation and
entailment machinery for equalities doesn't know about FDs. (I would
think that should lead to an error message rather than non-termination,
but oh well....)
Incidentally,
{{{
module ShouldCompile where
plus :: (a ~ (Int -> Int)) => Int -> a
plus x y = x + y
}}}
works fine.
I would expect that this would be fixed as soon as we implement FDs by way
of TFs.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1781#comment:1>
GHC <http://www.haskell.org/ghc/>
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